1. Field
The embodiments described herein relate generally to particle accelerators. More particularly, the described embodiments relate to particle accelerators used for treatment and/or imaging.
2. Description
A particle accelerator produces charged particles having particular energies. In one common application, a particle accelerator produces a radiation beam used for medical radiation treatment. The beam may be directed toward a target area of a patient in order to destroy cells within the target area by causing ionizations within the cells or other radiation-induced cell damage.
A conventional particle accelerator includes a particle source, an accelerator waveguide, an RF (radio-frequency) power source, and a bending magnet. The particle source may comprise an electron gun that generates and transmits electrons to the waveguide. The RF power source, which may comprise a magnetron or a klystron/RF driver, delivers an electromagnetic wave to a window built into the waveguide. The electromagnetic wave enters the waveguide through the window and oscillates within the waveguide. The oscillations accelerate the transmitted electrons through the waveguide. Finally, the bending magnet receives the accelerated electrons, filters them according to their energies, and emits them toward a target area.
Radiation treatment plans are designed to maximize radiation delivered to a target while minimizing radiation delivered to healthy tissue. However, designers of a treatment plan assume that relevant portions of a patient will be in a particular position relative to a particle accelerator during delivery of the treatment radiation. If the relevant portions are not positioned exactly as required by the treatment plan, the goals of maximizing target radiation and minimizing healthy tissue radiation may not be achieved. More specifically, errors in positioning the patient can cause the delivery of low radiation doses to tumors and high radiation doses to sensitive healthy tissue. The potential for misdelivery increases with increased positioning errors.
Conventional imaging systems may be used to determine a patient position prior to treatment according to a particular radiation treatment plan. For example, a radiation beam is emitted by a particle accelerator, passes through a volume of the patient and is received by an imaging system. The imaging system generates a two-dimensional portal image of the patient volume, which can be used to determine whether the patient is in a position dictated by the particular treatment plan.
A radiation beam used for imaging as described above delivers a radiation dose to the patient volume. The dose is preferably significantly less than a dose rate used for radiation treatment, but suitable to produce a satisfactory portal image. Low dose rates are particularly desirable if cone beam imaging is used to produce a three-dimensional image of the patient, since such imaging requires the acquisition of several two-dimensional portal images.
Conventional particle accelerators are unable to efficiently generate a radiation beam that provides a dose rate suitable for imaging at a given energy. Attempts have been made to output a radiation beam during beam formation for imaging purposes. These attempts are based on the low dose rates provided by a radiation beam while the beam is formed within an accelerator waveguide. For example, dose rates may be sufficiently low for imaging during a period while a beam forms within an accelerator waveguide. However, the dose rate over time is unstable and non-linear during the beam formation period. These characteristics are caused at least in part by beam loading and thermal deformation of the accelerator waveguide, both of which unpredictably change the resonant frequency of the waveguide during beam formation. Generation of a low-dose radiation beam can therefore be both difficult and unpredictable.